The problem is of course that necklines in this period are round and woven trim is straight and not very flexible. You can of course cut out a shaped piece of fabric, but if it's patterned it may look a little off when the pattern doesn't follow the line of the neck opening.
Anyway, that's not what I was going to write about, but how to apply straight, woven trim. In this case a four centimetres wide "gold" trim that I bought at Passamaneria Valmar in Florence this spring.
First, if you have a thin main fabric, like the tropical weight wool twill I'm using for this gown you probably want to reinforce the place where the trim will be placed. This will stop the stiff trim from distorting the fabric. I use scraps of fairly thick fulled wool for this. The photo is taken after I sewed the trim to the gown, but you get the idea.
I do the same when I make buttonholes on thin wool: apply strips of felted wool to provide stability.
When it has the right shape, iron it, with plenty of steam, and it might me a good idea to have a piece of cloth between the trim and the iron too. At least you won't get plastic on your iron if it melts. I pinned the lower edge of the trim first and then ironed the trim to the same shape as the neck opening, moving some of the gathers as needed.
Of course even the tiniest gathers will be visible when inspected closely, but with a good steamy ironing they are not easy to see from a couple of feets distance.
On this piece of trim I didn't place a fold at the shoulders, which you of course can do to. Then you get less gathers, but I thought that it shaped up so nicely even without a fold so I went for just gathering this time.